1. Field of the Invention
This application relates to strip perforating systems using a plurality of shaped charges in a wellbore to form perforations through which water, petroleum or minerals are produced, and in particular, the incorporation of shaped charges composed of an improved zinc alloy to improve creep resistance into the perforating system.
2. Description of the Prior Art
This invention is an improvement of the prior art of shaped explosive charges. The conventional zinc alloy that is used to manufacture standard oil field shaped charges is Alloy ZA-5. There have been few improvements on the material used to manufacture the casing of the shaped charges. This is largely due to the use of shaped charges that are encased in metal piping as it is lowered into the wellbore, thus protecting the charge from the harsh wellbore environment. However, due to the advantages of using capsule-exposed shaped charges, there is a need for an improved material for manufacturing the casing or capsule.
Other shaped charges employing various metal alloys are disclosed. For example, Aubry et al. (U.S. Pat. No. 4,922,825); Brauer et al. (U.S. Pat. No. 5,098,487); Mandigo (U.S. Pat. No. 4,958,569); Reese et al. (U.S. Pat. No. 5,656,791). In these inventions, the unique features disclosed are various compositions in specific parts of the shaped charge. In Aubry et al., the invention is an explosive charge incorporating a "coating" that acts as a projectile upon explosion of the charge. In Brauer et al., the invention is a metal liner which covers the open face of the explosive material and creates a perforation that does not leave behind a metal slug which can impede the flow of oil. In Mandigo, the invention is a metal liner as in Brauer et al., but using a wrought copper alloy. Finally, Reese et al. discloses an invention of a liner for a shaped charge made of a mixture of tungsten and powdered metal binder instead of copper as in previous disclosures. None of these prior art inventions use any unique mixture of metals or alloy compositions for the casing material. These prior art references are primarily directed toward the liners used in the shaped charges under consideration.
The conventional ZA-5 zinc alloy exhibits many properties that makes it a good choice for die casting charge cases. These properties include: (a) easily die-formed into shaped charge cases, (b) material vaporizes upon detonation of the charge, (c) high density material maintains the shaped charge performance, (d) acid soluble material, (e) inexpensive materials, (f) little machine work needed before loading charge, and (g) low cost process to manufacture the case. In any new material used to make shaped charges it would be desirable to maintain these properties.
There are, however, two important drawbacks to the conventional zinc alloy shaped charges: low yield strength, and low creep resistance. The low yield strength equates to lower performance shaped charges and low pressure resistance for capsule-exposed charges such as the charges in Shirley et al. (U.S. Pat. No. 5,638,901). Further, creep resistant qualities are important in perforating guns where the shaped charges are exposed to extreme pressures during placement in the wellbore. Thus, it is important to have a high creep resistance in the charge for maximum performance.
Further, the low creep resistance of the conventional materials causes poor performance of the shaped charges when used as exposed-capsule charges. When the case material creeps (moves) under stress, as when being lowered into the wellbore, the charge liner and explosive load relaxes and moves from its proper position. This creates and unpredictable and uncontrollable pattern of perforations within the formation surrounding the wellbore.
The only alternative inexpensive die material that currently works for exposed-shaped charges as in Shirley et al. is certain aluminum alloys. However, these have many disadvantages such as (a) requiring more expensive cold chamber process in its formation, (b) low density aluminum is inferior to high density zinc alloy, (c) a high temperature of vaporization which will not allow the casing of the shaped charge to disintegrate when the charge detonates, and (d) the aluminum alloys have a lower acid solubility.
The present invention is an improvement to zinc alloy shaped charges that solves the above mentioned problems with Zinc Alloy Z-5 and other aluminum alloys, while maintaining the beneficial properties of the Z-5 alloy. The new shaped charge utilizes a zinc alloy with additional copper and aluminum known as ACuZinc. This material is discussed in certain prior art references such as Rashid et al. (U.S. Pat. No. 4,990,310), and Hanna et al. (U.S. Pat. No. 5,509,728) for use in diverse environments, such as that of a brake system. The invention described in those patents is assigned to General Motors Corporation, and they own the trademarked name ACUZINC as well.
The ACuZinc alloy was designed primarily for use as a creep-resistant alloy in automobile brakes. The physical properties of the alloy were desirable for withstanding the high forces and temperatures that are generated in the gear mechanisms of anti-lock braking systems. A use in an explosive device where the alloy disintegrates was not disclosed nor envisioned.
The ACuZinc alloy has the advantage of being easy and inexpensive to manufacture. ACuZinc alloy can be formed by traditional die casting operation. Typically, molten metal is injected at high pressure into a fixed-volume cavity defining the shape of the product desired. The cavity typically has a water-cooling jacket to cool the casted product. The molten metal is injected into the cavity by a shot apparatus comprising a sleeve for receiving a charge of the molten metal and a plunger that advances within the sleeve to force the molten metal into the cavity.
Two types of casting apparatus exist. In a hot chamber apparatus, a shot sleeve is immersed in a bath of the molten metal. In a cold chamber apparatus, the molten charge is transferred into the shot apparatus from a remote holding furnace. Although shaped charges can be cast using both methods, the hot chamber method is preferred due to lower cost. Alloys that presently exist embodying the desired creep resistance must be made by the cold chamber method. The ACuZinc alloy has the advantage of being employable by both hot and cold chamber methods. Thus, using the ACuZinc alloy will lower the cost of shaped charges as well as increase the strength.
The proposed embodiment for the shaped charge would employ the ACuZinc alloy. The shaped charge is a typical Owen-capsule exposed charged like the Shogun NT charge as disclosed in Shirley et al. These capsules are unique from prior art (e.g., Aubry et al., U.S. Pat. No. 4922,825; Brauer et al., U.S. Pat. No. 5,098,487). Since the Owen-capsules are exposed to the wellbore environment when in use, they must be able to withstand extreme pressures and resistance while being lowered into the wellbore. The ACuZinc alloy is ideal for this application because of its high strength at high temperature, high creep resistance, acid solubility, ease of hot chamber die casting, low cost, and high density.